Batteries used in stand-alone power supply systems are commonly lead-acid batteries. However, lead-acid batteries have limitations in terms of performance and environmental safety. Typical lead-acid batteries often have very short lifetimes in hot climate conditions, especially when they are occasionally fully discharged. Lead-acid batteries are also environmentally hazardous, since lead is a major component of lead-acid batteries and can cause serious environmental problems during manufacturing and disposal.
Flowing electrolyte batteries, such as zinc-bromine batteries, zinc-chlorine batteries, and vanadium flow batteries, offer a potential to overcome the above mentioned limitations of lead-acid batteries. In particular, the useful lifetime of flowing electrolyte batteries is not affected by deep discharge applications, and the energy to weight ratio of flowing electrolyte batteries is up to six times higher than that of lead-acid batteries.
However, manufacturing flowing electrolyte batteries can be more difficult than manufacturing lead-acid batteries. A flowing electrolyte battery, like a lead acid battery, comprises a stack of cells to produce a certain voltage higher than that of individual cells. But unlike a lead acid battery, cells in a flowing electrolyte battery are hydraulically connected through an electrolyte circulation path. This can be problematic as shunt currents can flow through the electrolyte circulation path from one series-connected cell to another causing energy losses and imbalances in the individual charge states of the cells. To prevent or reduce such shunt currents, flowing electrolyte batteries define sufficiently long electrolyte circulation paths between cells, thereby increasing electrical resistance between cells.
Electrolyte is commonly supplied to, and discharged from a cell stack via external manifolds. Each cell has multiple inlets and outlets at capillary openings of the electrolyte circulation paths. Each external manifold is connected to the circulation paths of the cell stack using a delicate connection apparatus comprising an array of elastomer connection tubes. A typical 54-cell stack requires 216 elastomer connection tubes. Such a delicate connection apparatus is not only difficult to manufacture, but is also prone to damage during assembly and use. In order to reduce the likelihood of damage, internal manifolds formed within the casing of the battery have been developed in order to reduce the likelihood of damage.
FIG. 1 illustrates a diagram of a partial perspective view of a battery 10 constructed according to the prior art. The battery 10 includes a stack of cells 11 and an integral manifold 12. The integral manifold 12 includes a plurality of holes 13 through the integral manifold 12 to the capillary openings 14. A formwork 16 is then constructed around the cells 11. Pins 15 pass through the formwork 16 and through each hole 13 to the capillary openings 14. Once the formwork 16 has been constructed, a mould material (not shown) is injected into a cavity 17 via a fill aperture 18. Once the mould material has set, the pins 15 and the formwork 16 are removed and the fill aperture 18 and holes formed by the pins 15 on an outside of the mould are then plugged, forming a fluid connection between the capillary openings 14 and the internal manifold 12. Fittings (not shown) are then connected to the integral manifold 12 via a seal for connecting to the electrolyte flow.
However, using the mould design illustrated in FIG. 1, a bond between the mould material and the stack of cells 11 is relatively weak and can be a source of leaks. In addition, another source of leaks can be a connection between the fittings and the seal for connecting to the electrolyte flow.
Further, the relatively high pressures used in prior art injection moulding can be problematic when moulding adjacent relatively delicate materials. Typical moulding parameters for normal high density polyethylene (HDPE) parts include temperatures from 230 to 260 degrees C. and pressures from 30 to 60 bar. For insert moulding, where a separate part which may be of different colour or even different material is positioned inside the mould before moulding takes place, the pressure must be higher or the inserted part will not bond with the injected plastic. Thus typical pressures for insert moulding are 50 to 70 bar, and temperatures will be at the higher end of the above range to improve bonding. However, in a flowing electrolyte battery cell stack, because such a large part of the mould is made up of the inserted parts, and these parts are made of soft, low strength, and porous plastics, the above pressures can deform the inserted parts.
There is therefore a need to overcome or alleviate many of the above discussed problems associated with flowing electrolyte batteries of the prior art.